Optical substrate having alignment fiducials

ABSTRACT

A process comprising: (a) applying a stop etch mask to a substrate, the mask defining the location on the substrate of a groove for receiving a waveguide and one or more fiducials for positioning the optical device on the substrate relative to the waveguide; and (b) etching the substrate to define the fiducials and the groove, the groove being dimensioned to receive at least a portion of a waveguide and the fiducials enabling the optical device to be positioned on the substrate such that it is optically aligned with the waveguide.

RELATED APPLICATION

The application is based upon Provisional Application No. 60/238,815,filed Oct. 6, 2000, and hereby incorporated by reference.

FIELD OF INVENTION

This invention relates generally to optical substrates, and, moreparticularly, to optical substrates having alignment features foraligning optical components thereon.

BACKGROUND OF INVENTION

It is well known to align optical components on a silicon substrateusing register surfaces as mechanical stops. In this approach, anoptical device, such as a laser, is precisely aligned to a silicon wafersurface by butting a precision notch surface of the laser against amechanical stop or pedestal protruding from the silicon wafer boardsurface. The height of the laser or optical device is determined by theheight of a separate etched standoff positioned under the laser on thewafer surface. An optical fiber, intended to be optically aligned tothat laser, is registered to the silicon wafer board by bonding it to aprecision-etched V-groove. Because the optical components are aligned tothe silicon wafer board, they are aligned to each other. This alignmentprocess is referred to herein as “physical passive alignment” or“physical alignment.”

Although physical alignment offers relatively high accuracy in aligningoptical components on a substrate, the applicants have identified twomajor disadvantages of the approach which limit its accuracy. First, theformation of the V-groove is made traditionally by a crystallographicwet etch which is very sensitive to the orientation of the wafer crystaland is constrained in shape. Indeed, the sole purpose of thecrystallographic etch is to expose slow etching 111 silicon wafercrystal surfaces that then can be used for holding parts mechanically.Unfortunately, commercially-available silicon wafers are provided withorientation markings which have a significant tolerance, e.g., +/−0.5°.If an etching mask, for the purpose of etching a V-groove trench, isplaced on the surface with the trench out of crystal alignment by even0.50°, the trench will etch deeper than expected and the centerline ofthe trench will shift out of position making the substrate for holdingthe optical components slightly imprecise.

The second major disadvantage of the present process is that the maskfor defining the V-groove trench is not on the same mask level as themask used for defining the fiducials for positioning the laser. Thismeans that there is additional error introduced if the two separatemasks are not precisely aligned to each other's position in subsequentfabrication steps in the wafer board's fabrication.

Therefore, the applicants have identified a need for an opticalsubstrate having precision alignment features which are unaffected bycrystalline misalignment and avoid tolerance build up of multiple maskapplication steps. The present invention fulfills this need amongothers.

SUMMARY OF INVENTION

The present invention provides for an optical substrate having highlyprecise alignment features for receiving and aligning optical componentsthereon. The accuracy and precision realized by the substrate of thepresent invention is due to a single masking step which defines all thecritical alignment features on the substrate. Traditionally, this wasnot possible because the various alignment features were formed withtotally different processes requiring different conditions and separatemasking steps. Specifically, the V-groove is deep and thus required awet etching process, while the fiducials have planar surfacesperpendicular to the substrate surface and thus required a dry etchingprocess such as reactive ion etching (RIE). The applicants, however,have recognized that inductively coupled plasma (ICP) etching may beused both for deep etching and for vertical planar surfaces. This oneetching process allows for a single masking step which eliminates maskmismatches and tolerance buildup.

Additionally, the problem of crystallographic wafer orientation isobviated because the ICP process is an isotropic etching process meaningthat it has essentially no dependency on the wafer crystal orientation.In fact, it should even work for polycrystalline wafers which are castrather than grown epitaxially, and thus are much less expensive thancrystalline silicon.

Therefore, the present invention exploits ICP etching to prepare moreoptimized, higher precision substrates for passive alignment of opticalcomponents.

One aspect of the invention is a process of preparing a substrate usinga single making step to define the critical alignment features. In apreferred embodiment, the process comprises: (a) applying a stop etchmask to a substrate, the mask defining the location on the substrate ofa groove for receiving the waveguide and one or more fiducials forpositioning the optical device on the substrate relative to thewaveguide; and (b) etching the substrate to define the fiducials and thegroove, the groove being dimensioned to receive at least a portion of awaveguide and the fiducials enabling the optical device to be positionedon the substrate such that it is optically aligned with the waveguide.

Another aspect of the invention is a substrate made in accordance withthe process of the invention.

Yet anther aspect of the invention is an optical component substratehaving highly precise alignment features. In a preferred embodiment, thesubstrate comprises: (a) a groove for receiving a waveguide; and (b)fiducials for facilitating the alignment of an optical device on thesubstrate, wherein the tolerance of the alignment of the groove to thefiducials is less than ±0.2 μm. an optical subassembly.

Still another aspect of the invention is an optical subassemblycomprising the substrate of the present invention. In a preferredembodiment, the optical subassembly comprises: (a) an optical componentsubstrate comprising at least: (i) a groove for receiving a waveguide;(ii) fiducials for facilitating the alignment of an optical device onthe substrate, wherein the tolerance of the alignment of the groove tothe fiducials is less than ±0.2 μm. (b) a waveguide disposed in thegroove; and (c) an optical device aligned with the fiducials.

A further aspect of the invention is an optical package comprising theoptical subassembly of the present invention. In a preferred embodiment,the optical package is a transceiver.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a flow chart of the process of the present invention.

FIG. 2 is a perspective view of a substrate of the present inventionhaving precision alignment features.

FIG. 3 is a perspective view of an optical subassembly comprising thesubstrate of FIG. 2.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT

Referring to FIG. 1, a flow chart of the process of the presentinvention is shown indicating the steps for preparing a substrate withalignment features. As used herein, the term “alignment features” refersto contours in the substrate formed by etching which are adapted toreceive a waveguide and at least one optical device and align them onthe substrate such that they are optically coupled. It should beunderstood that optical coupling may be achieved directly by coaxiallyaligning the optical axes of the optical device and waveguide orindirectly by using a light bending device such as a prism to couple theoptical axes of the components. Typical alignment features include, forexample, a groove for receiving a waveguide, such as a fiber, andfiducials for providing a reference surface for placing an opticaldevice, such as a laser die, on the substrate in precise alignment withthe waveguide.

In step 101, a suitable substrate is provided for contouring alignmentfeatures thereon. The substrate has an x, y, z, orientation and asubstrate surface along an xz plane. As used herein, the terminologyrelated the x, y, z orientation is for illustrative purposes and shouldnot be used to limit the scope of the invention. The substrate surfacehas a specific point thereon about which the optical device andwaveguide are positioned so that they optically couple. The specificpoint typically corresponds to an optical axis of the optical device,such as the light emitting spot of an edge emitting laser. In step 102,a mask is applied to the substrate. The mask defines the location on anxz plane of the substrate for the alignment features. In step 103, thesubstrate is etched to define the fiducials and the groove. It isimportant to understand, that although the etching step is depicted inone process step in this flowchart, it is likely and even preferablethat the etching step be performed using a number of discrete sub-stepstargeted at different features of the substrate.

Once the etching is complete, a substrate is provided which has precisealignment features for receiving optical components. Such a componentmay be shipped to third parties for incorporation of the opticalcomponents therein or the process may be extended to include thepopulation of the substrate with optical components as depicted in steps104 and 105, which may be performed in any order. In step 104, theoptical device is disposed on the substrate and aligned with thespecific point on the substrate using the fiducials, either by physicalpassive alignment or visual passive alignment. In step 105, thewaveguide is disposed in the groove and is aligned to the specific pointon the substrate by registering it against surfaces defined by thegroove. At this point an optical subassembly is provided which may beincorporated into a larger optical package such as an amplifier,transmitter, receiver, or transceiver in step 106.

Each of the above steps are described in greater with respect to theFIGS. 2 and 3 which depict, respectively, a plain substrate havingalignment features and an optical subassembly populated with an opticaldevice and waveguide.

With respect to process step 101, the substrate 200 may be any materialsuitable for etching and formed to have a planar substrate surface 201in an xz plane. Suitable materials include, for example, silicon (eithercrystalline or polycrystalline wafers), silica, and ceramics such asalumina, aluminum nitride, and silicon carbide. A polycrystallinesilicon wafer is preferred from the standpoint of thermal conductivity,commercial availability, and low cost. Its unpredictable and imprecisecrystalline structure is of little concern since the etching used instep 103 is preferably an inductively coupled plasma (ICP) etchingprocess which is isotropic unlike wet etching which requires aparticular crystalline structure to etch in a predicatble fashion.Furthermore, since an ICP etching process does not rely on thecrystalline structure of the material, there is no need to invest timeand effort in determining the silicon's crystalline orientation.

It may be preferable to prepare the silicon substrate for etching byoxidizing its surface using techniques that are well known in the art toform a layer of oxide on the surface. It has been found that an oxidelayer of 5500 Å provides for good results, although those of skill inthe art may develop optimum thickness for particular applications.

With respect to step 102, the masking of the alignment features isperformed using conventional techniques with a single mask which definesthe location of the alignment features in an xz plane of the substrate.Since the single mask level can define all of the key critical alignmentfeatures, there is no error in aligning one mask level to another withsub-micron high positional precision. Thus, tolerance buildup and maskmisalignment are avoided. Furthermore, it is preferable to apply themask before the surface of the substrate is etched or contoured to anysignificant extent since masking is most accurate on a planar surface.Thus, a single mask which defines all of the critical optical alignmentfeatures is applied to the substrate at the point in which the marginfor error is the lowest.

With respect to step 103, the alignment features are etched into thesubstrate. In a preferred embodiment, different types of fiducials areetched to provide the optical device with passive x, y, z alignment onthe substrate. These fiducials may be used as a visual indicator toalign the optical device visually or they may be used as mechanical“stops” to align the optical device physically. Preferably, thefiducials are used for physical alignment due to the greater accuracyphysical alignment affords and due to its facilitation of bulk reflowprocesses for increased throughput.

In physical alignment, these fiducials are configured to contactregister surfaces on the optical device which are precise distances fromthe device's optical axis along the x, y, z axes. Such physicalalignment is well known in the art. Briefly, to control the opticaldevice's position along the y-axis, preferably one or more standoffs orfirst fiducials 205, 215 are used. In this particular embodiment, firstfiducials 205 are adapted to receive and support a laser 301, whilefirst fiducials 215 are adapted to receive and support a detector 302for the laser. The first fiducials have a first register surfaces 205 aand 205 a which are parallel to the substrate surface 201, i.e., in anxz plane.

To control the optical device's position along the x and z axes,preferably one or more second and third fiducials or pedestals are used,respectively. In this particular embodiment, second fiducials 206 and216 and third fiducials 207 and 217 are used for the laser 301 anddetector 302 respectively. The second and third fiducials have secondand third register surfaces 216 a and 217 a which are perpendicular tothe substrate surface 201. In one embodiment, the second and thirdregister surfaces lie, respectively, in an yz plane and an xy plane. Incertain embodiments, it may be preferable to use a set of the second orthird fiducials, i.e., two second fiducials 206 as shown, to prevent theoptical device from rotating in the xz plane.

The other alignment feature of particular interest in this applicationis the groove. The groove provides edges or walls which the waveguidecontacts to position the waveguide precisely with respect the specificlocation on the substrate. Although any conventional groove forreceiving a waveguide can used, in a preferred embodiment, a U-groove202 is formed in the substrate. A U-groove is preferred over a V-groovedue to its greater precision, especially with respect to positioning afiber 303 therein. Specifically, in a U-groove, the position of thefiber along the x and y axes is achieved by registering the perimeter ofthe fiber against the edges formed by the top portion of the U-groovewall and the top of the substrate. Since the walls of a U-groove areessentially parallel, the controlling parameter of the U-groove withrespect to the y axis positioning, i.e., width, remains constantregardless of the depth of the groove. On the other hand, a V-groove hasdivergent walls and thus the depth of the V-groove has a dramatic affecton the y-axis position of the fiber therein.

Unfortunately, in a U-groove, it is not possible to contact the fiber'sperimeter with the edges of the U-groove unless the U-groove is morenarrow than the fiber. Thus, it is not possible to lower the center ofthe fiber below the top edge of the U-groove. This is a significantproblem given the relatively large size of the waveguide compared to theoptical devices and thus the need to lower the waveguide's optical axisto that of the optical device's. The applicants, however, have overcomethis problem by etching a U-groove terrace 203 around the U-groove 202.This lowers the edges 219 formed by the U-groove and the effective top201 a of the substrate and thereby lowers the centerline of the opticalaxis of the fiber with respect to the substrate surface.

In addition to etching edges in the substrate for aligning the waveguidealong the x and y axes, preferably a mechanical stop 204 is etched toposition the fiber along the z-axis. The stop preferably has a fourthregister surface 204 a in the xy plane which contacts the end face ofthe fiber to position the end face a precise distance from the laser.

In etching the alignment features on the substrate, it is oftennecessary to etch deeply into the substrate. For example, the bottom ofthe U-groove needs to be deep enough to bring the optical axis of thefiber 303 to approximately the surface 201 of the substrate plus orminus the distance the optical axis of the laser 301 is offset from thesubstrate surface 201. Therefore, since a common optical fiber has adiameter of about 125 μm, a groove depth of about 62.5 μm ± the offsetof the optical device's optical axis from the substrate surface isrequired. In addition to deep etching, it is also often necessary toetch planar surfaces perpendicular to the substrate surface. These twofeatures have been opposing requirements historically. Specifically,while wet etching could produce deep etches, it was unable to producevertical surfaces. On the other hand, traditional RIE could producevertical surfaces, but it could not etch deeply (a traditional REIchlorine process has a practical etching depth limit of about 13 μm).Applicants have recognized that inductive coupling plaza (IPC) etchingcan reconcile these disparate requirements. IPC can not only etch deeplywithout a practical limit, but also etch vertical surfaces with highprecision. A preferred technique of IPC etching was developed by RobertBosch GmbH and apparatus for performing the process is commerciallyavailable through Zap PlasmaTherm. The process developed by Bosch isreferred to herein as the “Bosch process.”

The etching process is a subtractive process so in order to havealignment features rise above the surface it is necessary to removeeverything around that feature so that the surface is actually loweredrelative to the alignment feature. For this purpose, an etched field 210is employed as shown in FIG. 2. Unlike the earlier silicon wafer boardprocess based on chlorine RIE etching, the ICP process is sensitive toloading of the amount being etched at any time, so only areas around thecritical alignment features are removed. The remaining area is left atfull substrate thickness.

In the particular embodiment shown in FIG. 2, there are four etchedlevels plus the original wafer surface level. The deepest etch level,level 1, is at the bottom surface of the U-groove 202 a. This providesclearance for the bottom of the optical fiber, and ranges typically fromabout 60 to about 65 μm below the surface of the substrate. This etchalso defines the width of the groove. In a particular embodiment, anetch of this depth may also define the bottom 208 of the mirror wellused to reflect light up to a detector. The next deepest level, level 2,is the U-groove terrace 203. Its purpose is to set the height of thefiber in the U-groove (i.e., the fiber's position along the y-axis) andthus define the edges 219. The terrace 203 must be below the etchedfield 210 in height in order to set the centerline of the optical fiberin line with the optical axis of the optical device on the substrate200. The next deepest level, level 3 is the etched field 210 whichserves to open up the area around the fiducials, while still maintaininga significant amount of unetched silicon surface outside the etchedfield. The next deepest level, level 4, is the height for the laser anddetector standoff or first fiducials 205 and 215. This level is used toset the height of the chips, but also may provide clearance for theelectrical traces 209 and a controlled height for the solder 211 if theoptical device is an optoelectric device such as a laser orphotodetector. The highest level, level 5, is for the heights of thepedestals (or the second and third fiducials) and the mechanical stopfor the waveguide. This level represents the original wafer surface 201and remains unetched.

The following is an example of a particular process that can be used toprepare the substrate of FIG. 2. This example is offered forillustrative purposes and is not intended to restrict the scope of theclaimed invention. Indeed, one skilled in the art may recognizealternatives and optimization of this process.

1. Start Silicon Wafers 2. Thermal Oxidation (5500Å) 3. FirstLithography (fiducials and U-groove) 4. Oxide RIE 5. Resist Strip 6.Second Lithography (U-groove Protection) 7. Bosch Etch (Pit) 8. ResistStrip 9. Third Lithography (Standoffs) 10. BOE etch 11. Resist Strip 12.Forth Lithography (U-groove Protection) 13. Bosch Etch (Pit + Standoffs)14. Resist Strip 15. Nitride Deposition (1000Å) 16. Evaporate Ti/Ni/Au(500/500/500Å) 17. Fifth Lithography (Trace Plating) 18. Au Plate (6 μm)19. Resist Strip 20. Sixth Lithography (Solder Plating) 21. Au/Sn/AuPlate 22. Resist Strip 23. Seventh Lithography (Metal Etch) 24. MetalEtch (KI/I:I, BOE) 25. Resist Strip 26. Eighth Lithography (U-grooveTier) 27. Resistor Etch (KI/I) 28. Resist Strip 29. Ninth Lithography(U-groove Tier) 30. Nitride Etch 31. Bosch Etch (U-groove Etch) 32.Oxide Etch 33. Bosch Etch (U-groove Tier) 34. Resist Strip 35. Inspect36. Lap (Optional) Steps 20 & 21 assume that the metalizations on bothlaser and detector are at same height

It is worthwhile to note that step 3 of the process, known as firstphotolithography, includes both the patterning of the fiber groove andthe fiducials in which all critical optical alignments are made. Allother mask alignments that follow this are non critical positionally anddo not affect the precision of the part. In contrast, the prior artprocess has at least two separate critical alignment photolithography(known as first and third photolithography). These two separate maskshave to be aligned to each other within sub-micron (typically 0.1) inorder to minimize mask overlay error. This can be very difficult,especially once the substrate becomes contoured with various etchings.The process of the present invention has no mask overlay error.

It is also worthwhile to mention that the oxide preparation time isreduced because the ICP process works quite well with a thinner oxidemask. The oxide/silicon etch selectivity is a factor of 10 higher thantraditional RIE approaches, allowing the etch to be deeper whilemaintaining precision. Furthermore, the silicon etch rate is more than10 times faster than traditional RIE (some reports have it as much as 70times faster). These advantages are in addition to those alreadymentioned including independence from wafer crystal orientation,elimination of overlay error between separate masking levels, andelimination of extra masking and fabrication steps.

The result of this process is a prepared substrate 200 having alignmentfeatures of unprecedented accuracy. Because all of the criticalalignment is performed in a single step, tolerances in the position ofthe register surfaces of fiducials and groove of about ±0.1 to 0.2 μmcan be realized. Such tolerances represent a significant improvementover traditional physical alignment approaches which, at best, havetolerances of ±0.3 to 0.6 μm. At this point, the substrate is ready tobe populated with the optical devices 301, 302 and fiber 303 as shown inthe optical subassembly 300 of FIG. 3.

With respect to step 104, the optical devices 301, 302 are mounted tothe substrate using the fiducials to align them to the groove. Althoughthe optical device is depicted in FIG. 3 as a laser with a detector, theinvention is not limited to these devices and the term “optical device”as used herein broadly covers purely-optical or optoelectric componentsused in both passive and active devices. The term “passive devices,” asused herein, refers to any optical or optoelectric device thatmanipulates an optical signal but which does not impart energy into thesystem. Examples of passive devices include add/drop filters, arrayedwave guide gratings (AWGs), splitters/couplers, and attenuators. As usedherein, the term “active device” refers to any optical or optoelectricdevice that either converts signals between optical and electricaldomains or imparts energy into an optical system. Examples of activedevices include lasers, photodetectors, monitors, and opticalamplifiers. The optical device may be part of a larger assembly or itmay comprise subcomponents.

In a preferred embodiment, the optical devices 301, 302 are aligned byphysically abutting them against the fiducials. Such a technique isknown in the art. Although this technique is known, the ICP processallows for very deep etching so it is now possible to build pedestalsand standoffs that are tall enough to be useful for aligning lasershaving thick regrowth structures on their epi surface such as found inburied heterostructure lasers, large spot lasers, and lasers withgratings such as DFB's.

With respect to step 105, the fiber 303 is positioned in the groove 202such that it is optically coupled with the laser 301. Although a fiberis depicted as the waveguide in FIG. 3, the invention is not limited tothese devices and the term “waveguide” as used herein refers to anyoptical component that transmits optical signals and includes opticalfiber and planar waveguides. The waveguide may be a discrete componentthat is mounted within the groove or it may be deposited in the grooveand formed in situ. Preferably the waveguide is a fiber.

Techniques for mounting a fiber in a groove are well known and includemetalizing the fiber and soldering the fiber to the groove, or adheringthe fiber to the groove using epoxy. The edges 219 defined by theU-groove 202 and U-groove terrace 203 align the fiber along the x and yaxes. The fiber stop 204 is used as the fourth register surface to alignthe fiber along the z-axis. Alternatively, the z-axis alignment of thefiber may be done actively if desired.

The optical subassembly 300 of FIG. 3 is suitable for incorporation intovarious devices such as multiplexers, amplifiers, transmitters,receivers, transceivers, sensors, switching equipment, and computersusing optical switching.

1. A process for preparing a substrate having alignment features tofacilitate passively mounting components on said substrate having an x,y, z, orientation and a substrate surface along an xz plane, saidsubstrate having a specific point thereon , said process comprising:applying a stop etch mask to said substrate, said mask defining thelocation on said substrate of one or more fiducials for positioning adevice on said substrate relative to said specific point; and etchingsaid substrate to define said fiducials using an inductively coupledplasma etching process, at least one of said fiducials comprising aplanar surface substantially perpendicular to said substrate surface,said planar surface being adapted to act as a register surface to enablesaid device to be positioned on said substrate such that it is in aprecise position relative to said specific point.
 2. The process ofclaim 1, wherein said inductively coupled plasma etching process is aBosch process.
 3. The process of claim 1, wherein said device is anoptical device and wherein said mask also defines the location on saidsubstrate of a groove for receiving said waveguide, and wherein saidgrove is also etched using an inductively coupled plasma etchingprocess.
 4. The process of claim 3, wherein said groove is deeper than13 μm from said substrate surface.
 5. The process of claim 4, whereinsaid groove is a U-groove.
 6. The process of claim 5, wherein saidU-groove has a bottom at about 60 to about 65 μm from said substratesurface.
 7. The process of claim 6, wherein etching said U-groovecomprises etching a U-groove terrace, said U-groove and U-groove terracedefining edges for receiving a waveguide.
 8. The process of claim 3,wherein a first fiducial defines a first register surface a firstcertain distance from said substrate surface alone said y-axis, a secondfiducial defines a second register surface a second certain distancefrom said specific point along said x-axis, a third fiducial defines athird resister surface a third certain distance from said specific pointalone said z-axis, and wherein said groove, and said first, second andthird fiducials are located on said substrate using the same mask, andwherein a mechanical stop has a fourth register surface a fourth certaindistance from said specific point along said z-axis, said mechanicalstop being adjacent to said groove and adapted to contact a waveguide insaid groove to position it from said specific point along the z-axis. 9.The process of claim 3, wherein a mechanical stop has a fourth registersurface a fourth certain distance from said specific point along saidz-axis, said mechanical stop being adjacent to said groove and adaptedto contact a waveguide in said groove to position said waveguide fromsaid specific point along the z-axis.
 10. The process of claim 3,wherein said waveguide is a fiber.
 11. The process of claim 3, wherein,when said optical device is in said precise position, it is opticallyaligned with said wave guide.
 12. The process of claim 1, wherein saidsubstrate is form from a materials selected from the group consisting ofpolycrystalline silicon, silica, and ceramics.
 13. The process of claim1, further comprising etching an etched field encompassing saidfiducials.
 14. The process of claim 1, wherein a first fiducial definesa first register surface a first certain distance from said substratesurface along said y-axis, a second fiducial defines a second registersurface a second certain distance from said specific point along saidx-axis, a third fiducial defines a third register surface a thirdcertain distance from said specific point along said z-axis, and whereinsaid groove, and said first, second and third fiducials are located onsaid substrate using the same mask.
 15. The process of claim 1, furthercomprising disposing an optical device on said substrate in a certainposition relation with respect to said fiducials.
 16. The process ofclaim 15, wherein optical device is disposed on said substrate byvisually aligning said optical device to said fiducials.
 17. The processof claim 15, wherein said optical device is disposed on said substrateby physically contacting said optical device with said fiducials. 18.The process of claim 15, wherein further comprising disposing awaveguide in said groove.
 19. The process of claim 1, wherein thetolerance of the alignment of said groove to said fiducials is less than±0.2 μm.
 20. The process of claim 1, wherein said optical device is atleast one of a laser, photodetector, or monitor.
 21. The process ofclaim 1, further comprising: effecting the positioning of said deviceagainst said register surface.
 22. The process of claim 21, wherein saiddevice is within ±0.2 μm of said precise position.